The use of thick-film conductors as components in hybrid microelectronic circuits is well known in the electronics field. An important application of patterned electrically-conductive layers is in the automobile industry, and particularly in the manufacture of windows which can be defrosted and/or demisted by an electrically-conductive grid permanently attached to the window and capable of producing heat when powered by a voltage source. The conductive grid generally comprises a series of tracks (or “hot-lines”) which are spaced regularly across one surface of the window, usually horizontally, between two “bus-bars” on opposing sides of the window, which are usually disposed vertically. The tracks and bus-bars are normally made from the same composition. Conductive compositions may also be used in various other applications, including printed circuits and heating elements generally, for instance, as base plates in hot water heating appliances. There is a general need within the electronics and electrical industry for lower-cost heating elements, particularly screen-printable heating elements.
Conventional compositions for the manufacture of such components take the form of a paste-like solid-liquid dispersion, where the solid phase comprises finely divided particles of a noble metal or a noble metal alloy or mixtures thereof and an inorganic binder, dispersed into a liquid vehicle. The inorganic binder is typically a glass or glass-forming material, such as a lead silicate, and functions as a binder both within the composition and between the composition and substrate onto which the composition is coated. Due to environmental considerations, the use of lead-containing binders is becoming less common and lead-free binders such as zinc or bismuth borosilicates are now often employed. The inorganic binder, also known as a frit, is considered a key component in conventional compositions.
Additional materials may be added in small quantities (generally less than about 3% by weight of the composition) to modify the properties of the composition and these include staining agents, rheology modifiers, resistivity modifiers, adhesion enhancers and sintering modifiers.
The consistency and rheology of the composition is adjusted to the particular method of application which may comprise screen-printing, brushing, dipping, extrusion, spraying and the like. Typically, screen-printing is used to apply the composition. The pastes are usually applied to an inert substrate, to form a patterned layer. The thick-film conductor layer is normally dried and then fired usually at temperatures between about 400 and 700° C., typically 600-700° C., to volatilize or burn off the liquid vehicle and sinter or melt the inorganic binder and the metal components. Direct wet-firing, i.e. wherein the thick film layer is not dried before firing, has also been used to generate the patterned layer.
In the manufacture of automotive backlights, there is typically an enamel layer coated around the periphery of the backlight, and it is in this area which the bus-bars are normally printed. As used herein, the term “enamel” refers to the layer applied to the surface of a substrate (typically glass) or part thereof onto which a conductor composition is applied. The enamel is a dispersion of a glass or glass-forming frit or binder (as described herein) in powder form in an organic carrier vehicle, normally with additional fillers and/or opacifiers and/or colorants. The enamel is typically colored black in order to provide an obscuration band around the periphery of the backlight. This is done primarily to provide protection against UV attack by sunlight on the adhesive that is used to glue the backlight into the car, but also for cosmetic or decorative purposes. The glass powder of the enamel is designed to soften and flow at the firing temperatures of the backlight to form a film that adheres to the surface of the substrate. The rheological characteristics of the enamel are determined in advance depending on the selected firing temperature of the backlight.
In practice, firing to effect sintering of the conductive pattern is effected in the same stage of manufacture as the firing of the backlight to shape it into its desired form and the firing to effect sintering of the enamel. The process of manufacture therefore comprises the following steps:
(i) printing the enamel composition onto the glass substrate, typically by a screen-printing technique, and then curing the composition by UV-radiation or drying at about 100-200° C. to drive off the solvent;
(ii) printing the conductive composition and optionally drying to drive off solvent; and
(iii) firing the coated glass substrate to effect sintering of the layers and forming of the backlight, optionally with a rapid cooling step to produce a toughened glass substrate, in accordance with conventional methods known in the art.
It is also necessary to connect the conductive pattern to the other components of the electronic circuit, such as the power source, resistor and capacitor networks, resistors, trim potentiometers, chip resistors and chip carriers. This is generally achieved by using metal clips, typically comprising copper, which are soldered either directly adjacent to or on top of the conductive layer, usually in the bus-bar sections of the pattern. Where the clips are soldered on top of the conductive layer, attachment is either directly onto the conductive pattern itself or onto a solderable composition which is overprinted onto the pattern (an “over-print”). An over-print is generally applied only in the region of the conductive pattern to which the metal clips are attached by solder, which region is generally referred to as the “clip area”. The ability to solder onto the electrically-conductive layer is an important parameter in the manufacture of heating elements since it removes the requirement for an over-print. However, the inorganic binder, which is important for binding the paste onto the substrate, can interfere with solder wetting and result in poor adhesion of the soldered metal clips to the conductive layer. The requirements of high substrate adhesion and high solderability (or adhesion of the metal clips to the conductive pattern) are often difficult to meet simultaneously. It is particularly important to ensure high substrate adhesion in the clip area since it is this area of the conductive pattern that is subjected to the most stress.
It is desired to decrease the visibility of the conductive pattern on the automotive backlight, particularly by decreasing the width of the conductive strips that form the conductive pattern. Typically the width of the conductive strips is about 1 mm in conventional demisting elements. In addition, it is also desired to maintain a high conductivity and low resistivity. However, it is not desirable to significantly increase the height of the conductive pattern above the surface of the glass backlight, which in conventional backlights is typically about 10 μm. It is therefore desired to reduce the cross-sectional area while retaining conductivity, and one way of achieving this is to increase the fired density of the material in the conductive pattern.
However, there is an upper limit to the concentration of solids for a conductive paste, which is suitable for manufacture of a conductive pattern. When the amount of solids in the dispersion exceeds this limit, the paste becomes difficult to handle in the processes used to manufacture the pattern. Moreover, an increased solids fraction in compositions comprising a conductive component and a frit component in conventional proportions has been found to lead to cracking of the enamel during the firing stage of the manufacture of the backlight.
It is an object of this invention to provide an economical electrically-conductive coating composition suitable for the manufacture of an electrically-conductive pattern, particularly a pattern having narrower width tracks, which exhibits good adhesion to the substrate and avoids or minimizes one or more of the above-mentioned disadvantages, particularly cracking of the enamel on an enamel-coated substrate. It is a further object of this invention to provide an economical electrically-conductive coating composition suitable for the manufacture of an electrically-conductive pattern, particularly a pattern having narrower width tracks, which exhibits good adhesion to the substrate whilst minimizing the cracking of the enamel, and which exhibits high conductivity and low resistivity.